(1) Field of the Invention
This invention is directed to the manufacture of very short inorganic glass fibers and mineral wool fibers with low shot content in a controlled range of aspect ratios. Aspect ratio may be defined as the ratio of fiber length to fiber diameter and is expressed as a dimensionless number. Shot is generally formed from cooled slag which has failed to be fully attenuated in the mineral wool manufacturing process. The short glass or mineral wool fibers can be used as reinforcing agents in plastics.
(2) Description of the Prior Art
Mineral wool fibers have been manufactured for a long time and are well known in the art. There are two commercial methods in current use for making mineral wool fibers. One of these methods is performed on an apparatus which uses a single dish-shaped rotor with steam attenuation to form the fibers. The rotor may be in a vertical or horizontal plane. Typical apparatus of this type is shown in the following U.S. Patents: No. 3,022,538, issued on Feb. 27, 1962 to C. B. Setterberg, No. 2,328,714, issued on Sept. 7, 1943 to D. C. Drill, and No. 2,944,284, issued on July 12, 1960 to W. T. Tillotson et al. The other convention method uses multiple rotors which hurl a molten stream of liquid melt against their outer rims in sequence to form the mineral wool fibers. Typical apparatus of this type is shown in the following U.S. Patents: No. 3,045,279 -- W. K. Hesse, issued July 24, 1962; No. 2,991,499 -- H. E. Holcomb, issued July 11, 1961, and No. 2,561,843 -- J. E. Coleman, issued July 24, 1941.
It should be noted that while some of these patents disclose apparatus for separating shot from fibers, none of them disclose a method of producing a very short mineral wool fiber. Rather the teachings of disclosures are directly opposed to that of shortening the fiber. Generally, these fibers are used in the form of long fibers which can be intertwined to form a nonwoven mat or held together by a binder to form a rigid board. Under these prior art conditions, it is desirable to have the fibers relatively long.
More particularly, mineral wool fibers have heretofore been used as heat insulation material in paper-enclosed batts, as reinforcing agents in spray-on heat insulation, or as fibers in rigid acoustical panels and tiles. For all of these applications, there is no need to shorten the length of the fibers as they are formed. In fact, it is generally preferred that the fibers be as long as possible with a large aspect ratio.
In many applications, efforts have been made to separate shot from mineral wool fibers.
Conventionally an air elutriation method is used to separate shot from fibers. This method usually has a stream of air which moves the fibers in an upward arc. It is well known that the shot particles have a much higher weight-to-length ratio than the fibers which are generally long and slender. The air stream not only separates the shot from the fibers because of the difference in response of the shot and the fibers but the air also, to some extend, breaks some of the shot away from the fibers to which it is attached.
In addition, the fibers and shot may be separated by using water as the separating medium. However, this latter method requires an additional drying step which makes the process less attractive than air elutriation.
The method of producing long glass fibers from a molten mass has long been known. Generally, a mass of glass marbles are melted in a heating unit and fine filaments of glass are extruded through small holes in the bottom of the heating unit. These filaments are then collected on spools or in an unwoven mat. Often the filaments are accumulated into bundles which are held together by a binder to give added strength. It is also known to cut or chop the fibers into shorter lengths.
It has also long been known to use defibrating or refining apparatus for reducing wood or cellulosic chips to individual fibers. In the defibrating or refining apparatus, wood chips are rubbed against one another until the result is a mass of long individual cellulose fibers which then can be felted into paper or fiberboard. However, in the process, it is desirable to keep the fibers as long as practicable, as very short fibers are useless for making paper or fiberboard.